In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. A piezoelectric injector 12 and its associated control system 24 are shown schematically in FIG. 1.
The piezoelectric injector 12 includes a piezoelectric actuator 16 that is operable to control the position of an injector valve needle 17 relative to a valve needle seat 18. The piezoelectric actuator 16 includes a stack 19 of piezoelectric elements, having the electrical characteristics of a capacitor. The stack 19 may be charged or discharged by application of a differential voltage to positive and negative terminals of the actuator 16, which causes the stack of piezoelectric elements to expand or contract. The expansion and contraction of the piezoelectric elements is used to vary the axial position, or ‘lift’, of the valve needle 17 relative to the valve needle seat 18.
The piezoelectric injector 12 is controlled by an injector control unit 22 (ICU) that forms an integral part of an engine control unit 24 (ECU). The ICU 22 typically comprises a microprocessor 26 and memory 28. The ECU 24 also comprises an injector drive circuit 30, to which the piezoelectric injector 12 is connected by way of first and second power supply leads 31, 32.
Typically, fuel injectors are grouped together in banks of one or more injectors, and each bank of injectors is selectably connected to the drive circuit 30 for controlling operation of the injectors.
In a so-called ‘discharge to inject’ injector, in order to initiate an injection event the injector drive circuit 30 causes the differential voltage applied to the injector 12 to transition from a high voltage (typically 200V) at which no fuel delivery occurs, to a relatively low voltage (typically −55V), which causes the valve needle 17 to lift away from the valve needle seat 18.
Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system, which could consequentially result in a catastrophic failure of the engine. Diagnostic systems for detecting short circuit faults in piezoelectric actuators of piezoelectric injectors are disclosed in applicant's co-pending patent applications EP 1843027, EP 1860306, EP 06256140.2, and EP 07252534.8, EP 07254036.2 the contents of each document being incorporated herein by reference.
Five main types of short circuit fault exist:
i) a short circuit between the terminals of a piezoelectric actuator; otherwise referred to as a ‘stack terminal’ short circuit;
ii) a short circuit from the positive terminal of a piezoelectric actuator to a ground potential; the positive terminal is also referred to as the ‘high’ terminal, and
this type of short circuit is generally referred to as a ‘high side to ground’ short circuit;
iii) a short circuit from the negative terminal of a piezoelectric actuator to a ground potential; the negative terminal is also referred to as the ‘low’ terminal, and this type of short circuit is generally referred to as a ‘low side to ground’ short circuit;
iv) a short circuit from the positive terminal of a piezoelectric actuator to a non-ground or ‘battery’ potential; this type of short circuit is generally referred to as a ‘high side to battery’ short circuit; and
v) a short circuit from the negative terminal of a piezoelectric actuator to a non-ground or ‘battery’ potential; this type of short circuit is generally referred to as a ‘low side to battery’ short circuit.
It is to be appreciated that a non-ground or battery potential refers to a voltage potential which is not ground, i.e. zero volts. Typically, this may be any low voltage derivable from the voltage supply or battery. These types of short circuit are referred to as high side or low side ‘to battery’ for simplicity. However, it does not exclusively refer to direct shorts to the battery terminal or potential.
Different techniques and methods are employed in the above-referenced co-pending patent applications in order to identify faulty injector banks. However, it has previously not been possible to identify individual faulty injectors 12 that are short circuited, because of the risk of ‘charge sharing’ between faulty and non-faulty injectors. Charge sharing occurs when a non-faulty injector 12a, 12b is selected causing it to discharge into a faulty injector 12a, 12b, and has previously prevented diagnostic techniques from being able to determine which of the individual injectors 12a, 12b are faulty.
An other problem associated with charge sharing is the risk that an uncontrolled injection could occur. If a low resistance short circuit were to occur it is possible that the faulty injector could fully discharge in a very short period of time. In so-called ‘discharge to inject’ systems, this results in the injector valve needle 17 lifting relative to the valve needle seat 18, and this could result in an increased volume of fuel and an uncontrolled injection. This could potentially cause damage to the engine if too much fuel is injected. In addition, the actuators could be damaged if uncontrolled currents are permitted to flow following a stack terminal short circuit.
Even if the short circuit is of a sufficiently high resistance to not cause engine or actuator damage, the performance of the engine may be adversely affected if a short circuit were to go undetected, and may result in undesired levels of fuel delivery and emissions.
Since it has not been possible to identify individual faulty injectors, the recovery action on detection of the fault is to shut down the entire injector bank. It is then necessary to carry out time-consuming tests during engine servicing to identify the faulty injector. These tests may not be conclusive and in some cases non-faulty parts may be replaced unnecessarily.
An aim of the invention is therefore to provide a diagnostic tool that is capable of detecting individual injectors which are short circuited and a method of operating the diagnostic tool.